Only run install-debian.wheezy.sysroot.py once during runhooks

Only run install-debian.wheezy.sysroot.py once during runhooks

This script was previously run three times, once for
each arch, with two of the three always being no-ops.

Instead just run the script once and have the script
figure out which image to download.

Second attempt at landing this.  First attempt caused
failures on windows builders:
 https://codereview.chromium.org/834133003

TEST=trybots + 'gclient runhooks' locally

Committed: https://crrev.com/d4eff31d57ec1be91736ba32f56dda2c8edfb195
Cr-Commit-Position: refs/heads/master@{#310753}

[Sam Clegg, 2015-01-08 11:36:30]

sbc@chromium.org changed reviewers:
+ thestig@chromium.org

[Sam Clegg, 2015-01-08 11:36:30]

The first attempt fail on the main waterfall but worked on the trybots:

http://build.chromium.org/p/chromium/builders/Win/builds/27781

I can't figure out how but it looks like detect_host_arch must have been
reporting something other than x64, ia32 or
arm.

Fixed by returning earlier (before arch detection) on non-linux.

[Lei Zhang, 2015-01-08 21:48:15]

lgtm

[Sam Clegg, 2015-01-09 10:38:34]

The CQ bit was checked by sbc@chromium.org

[commit-bot: I haz the power, 2015-01-09 10:38:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/843643003/40001

[commit-bot: I haz the power, 2015-01-09 11:14:58]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-01-09 11:15:45]

Patchset 3 (id:??) landed as
https://crrev.com/d4eff31d57ec1be91736ba32f56dda2c8edfb195
Cr-Commit-Position: refs/heads/master@{#310753}

[Sam Clegg, 2015-05-13 21:29:20]

Patchset #4 (id:60001) has been deleted